Size effects on polarization in epitaxial ferroelectric films and the concept of ferroelectric tunnel junctions including first results

摘要

Extrinsic and intrinsic size effects on the spontaneous polarization of epitaxial ferroelectric films are discussed. The extrinsic effect of electrostatic origin is attributed to the presence of nonferroelectric subsurface layers in the film. Theoretical studies of this depolarizing-field effect are reviewed. It is concluded that, for perovskite ferroelectrics sandwiched between electrodes with a perovskite structure, the depolarizing-field effect on the static properties should be negligible. The extrinsic size effect is also attributed to the thickness dependence of the film in-plane lattice strain S_m, which is due to the generation of misfit dislocations in the epitaxy. Variation of the film polarization with the misfit strain S_m is described by a nonlinear thermodynamic theory, which allows for the mechanical film/substrate interaction. The intrinsic effect of the film surfaces, which is associated with spatial correlations of the ferroelectric polarization, is simultaneously taken into account via the concept of extrapolation length δ. It is shown that, in films grown on compressive substrates (S_m < 0), the strain-induced increase of the mean polarization prevails over the negative intrinsic size effect (δ > 0). As a result, well below the transition temperature, ferroelectricity may be present even in nanometer-thick epitaxial layers. Motivated by this result, we propose the concept of ferroelectric tunnel junction. First results on tunnelling through ultrathin barriers of perovskite ferroelectrics are presented.